Instrumentation for use in spectroscopy of charged particles makes use of electrons or ions which are emitted from a substance after being bombarded or irradiated with electrons or ions from a source such as an electron gun. A technique to which the invention is particularly directed is known as Auger electron spectroscopy. In this technique, a target sample material is placed in a vacuum, usually below 10.sup.-7 Torr, and upon being bombarded with electrons from some source, such as an electron gun, the sample gives off a variety of emissions. Among these are X-rays, secondary electrons, and reflected primary electrons from the source. The emissions include Auger electrons (a particular class of secondary electrons) in the manner which is well known in the literature.
In the art of Auger electron spectroscopy, as taught for example in U.S. Pat. No. 4,205,226 (Gerlach), of the present assignee, instruments making use of cylindrical mirror analyzers ("CMA") are known which analyze the energy and the energy spectrum of Auger electrons emitted by the sample material. Such instruments operate by injecting the diverging electrons into a radial electric field produced between a pair of coaxially mounted electrode cylinders held at different electric potentials. Auger electrons injected from the sample into the radial electric field between the cylindrical electrodes are deflected by the field back toward the common axis of the electrodes. Electrons of a predetermined energy are thereby brought to a focus. By positioning a collector apparatus at this focus, electrons of a predetermined energy are selected and detected. By sweeping the voltage impressed across the cylindrical electrodes through a range of values, and detecting as a function of these applied potentials such electrons as are collected, the energy spectrum of the injected electrons may be plotted and determined.
As disclosed in the aforementioned patent, it is advantageous to utilize a detector system that is off-axis, for example annular, because the central axis is used for the primary beam to the target. Also, magnetic lenses used for the electron gun deflect paths of electrons being detected, thus interfering with precision measurements. German Patent Application No. 27 05 430 similarly discloses off-axis detection in several configurations and electron paths, representing efforts to increase sensitivity. U.S. Pat. No. 4,100,409 teaches bringing the electron gun beam from off-axis, eliminating magnetic lenses, and then utilizing an on-axis detector system to receive electrons from the cylindrical analyzer.
In an article "A Cylindrical Mirror Photoelectron Spectrometer With Position-Sensitive Detector Used For X-Ray Analysis" by O. Benka, Nuclear Instruments and Methods, 203, 547-550 (1982) there is described a CMA with a position sensitive detector positioned off-axis for multichannel detection of electrons leaving the CMA, to increase instrument sensitivity. The high precision detector useful for such an application is a channel plate detector of the type disclosed in an article "Microchannel Plate Detectors" by J. L. Wiza, Nuclear Instruments and Methods, 162, 587-601 (1979). Such a detector includes a ceramic plate formed with a large number of microchannels therethrough, each microchannel being an electron multiplier of incident electrons or ions.
The channel plate detector is quite effective but, because of its inherent construction, has generally been available commercially as a flat ceramic plate with a planar electron entry surface. Recently curved plate technology has become available on a limited basis but is extremely expensive. Therefore, Benka placed such a channel detector plate in only a small (40 degree) azimuthal portion of the 360 degree CMA. He further utilized a 6 degree polar range (12 degrees is actually more standard), and reported measuring 6.5% of the electron energy spectrum with 0.3% resolution. This result was actually an 18 times decrease in angular acceptance. This could be improved upon by utilizing a ring of such plates, but such an assembly would be a cumbersome and costly solution and still may not provide optimum sensitivity.